Asphalt, also known as bitumen or tar, is a highly viscous liquid or semi-solid form of petroleum. Asphalt is classified by material scientists as a pitch (viscoelastic polymer) that does not have a sharply defined melting point.
Asphalt waste, also referred to as bituminous waste, is a composition that includes asphalt and some other material that is not readily usable “as is”. Examples of asphalt waste include (but are not limited to) scrapped or left-over construction products that include asphalt, and tar or oil sands (collectively referred to as “tar sands” herein) that are a combination of bitumen, water, and other solids. Asphalt waste as used herein also encompasses compositions that include instead of asphalt highly viscous, essentially solid (at room temperature) petroleum oils such as heavy fuel oils or intermediate fuel oils that have an asphalt-like consistency at room temperature
Asphalt is used in construction products, such as asphalt cement for paving, coatings for sealing and insulation, and in products utilizing asphalt's waterproofing capabilities.
Asphalt is commonly used in the United States in the production of asphalt cement for paving road surfaces. Asphalt cement is made by heating asphalt to an elevated temperature and mixing the heated asphalt with aggregates (stone, sand, gravel, and the like). The asphalt acts as a binder that holds the aggregates together after the heated mixture has cooled. Asphalt concrete typically contains about 5% asphalt by weight.
The roofing industry makes wide use of asphalt to take advantage of asphalt's waterproofing ability. Mastic asphalt is similar to asphalt cement but has a higher asphalt content (typically about 7% by weight). Mastic asphalt is heated and spread on flat roofs to form a waterproof membrane. Asphalt is also used in the manufacture of asphalt roofing shingles. Asphalt is either mixed with a base material or coats a base material to form a waterproof shingle. Recycling asphalt material from shingles and other roofing material is very expensive due to the presence of extraneous material like fiber board, rubber, and other materials.
One of the largest sources of waste asphalt is naturally occurring tar sands found in large parts of North America, particularly in Canada and the United States. Oil can be extracted from tar sands. Standard processes to extract oil from tar sands include surface mining of tar sand deposits and heating tar sand deposits in situ to liquefy the bitumen. Extracting oil from tar sands utilizing these standard processes is complicated and has high environmental and energy costs. This is demonstrated for example, by the process described in Rennard, et al. U.S. Pat. No. 8,974,661.
Recycling asphalt cement is desirable for reusing both the asphalt and aggregates. According to the US Department of Transportation Federal Highway Administration, about 81% of waste asphalt cement is recycled and reused for paving. Recycling waste asphalt shingles is also desirable. Recycled asphalt shingles is a common component used in manufacturing asphalt cement. However, not all types of asphalt shingles can be used in making asphalt cement.
Even with recycling efforts a substantial amount of asphalt waste used for construction and paving end up in landfills. But asphalt waste does not efficiently degrade and therefore consumes much landfill space that becomes unavailable for treating biodegradable wastes.
Asphalt waste may be recycled by heating the waste sufficiently for the asphalt to melt and flow from the remainder of the waste. Often the asphalt waste is heated in a furnace to temperatures exceeding 750 degrees Fahrenheit.
The separation of the asphalt from the remainder of the asphalt waste also enables reuse of the remaining material. But recycling asphalt by melting the asphalt is expensive and energy intensive, requiring large amounts of water and fuel. Burning fuel to heat the asphalt is not a carbon-neutral process and produces large amounts of greenhouse gases. The high cost of recycling works to limit recycling of many types of asphalt waste.
Thus there is a need for an energy efficient method for recycling or processing asphalt waste that does not require melting the asphalt, and can be used for recycling or processing many different types of asphalt waste.